Method of correcting rotational motion error in SAR and ISAR imagery

Abstract

A method of correcting rotational motion error in SAR and ISAR imagery includes shifting the center of the image to zero doppler and thus to the center of rotation of the target being imaged, selecting a dominant scatterer at an off-center range cell to provide a phase reference, calculating the rotational motion error function from the phase of the selected scatterer, using the rotational motion error function to interpolate raw radar return image data between the original unequal angular samples to obtain equal angular samples of the target reflectivity function, and performing an image quality check and producing a value which is a relative measure of focus of the image. Also, the method includes repeating the steps starting with selecting a new dominant scatterer as a phase reference. Further, the performing step of the method includes storing and comparing the relative measures of focus provided by repeating of the steps to arrive at the corrected image which is the most focused and then saving the corrected image determined to be the the most focused.

Having thus described the invention, what is claimed is: 
     
       1. A method of correcting rotational motion error in SAR and ISAR imagery, comprising the steps of: shifting the center of the image of a target to zero doppler and thus at the center of rotation of the target being imaged;   selecting a dominant scatterer in a range cell to provide a phase reference;   calculating the rotational motion error function from the phase reference provided by the selected scatterer;   interpolating raw radar return image data between the original unequal angular samples by using the rotational motion error function to obtain equal angular samples of a reflectivity function for the target; and   performing an image quality check to produce a value which constitutes a relative measure of focus of the corrected image.   
     
     
       2. The method of claim 1 further comprising: repeating the steps starting with selecting a new dominant scatterer as a phase reference.   
     
     
       3. The method of claim 2 wherein said performing includes producing a plurality of values constituting relative measures of focus corresponding to the repeating of the steps starting with selecting a new dominant scatterer. 
     
     
       4. The method of claim 3 wherein said performing includes: storing the relative measures of focus produced by said repeating of the steps; and   comparing the relative measures of focus to determine the corrected image which is the most focused.   
     
     
       5. The method of claim 4 further comprising: saving the corrected image determined to be the most focused.   
     
     
       6. A method of correcting rotational motion error in SAR and ISAR imagery, comprising the steps of: shifting the center of the image of a target to zero doppler and thus at the center of rotation of the target being imaged;   selecting a dominant scatterer in a range cell to provide a phase reference;   calculating the rotational motion error function from the phase reference provided by the selected scatterer;   interpolating raw radar return image data between the original unequal angular samples by using the rotational motion error function to obtain equal angular samples of a reflectivity function for the target;   repeating the steps starting with selecting a new dominant scatterer as a phase reference; and   performing an image quality check at the end of each repeating of the steps to producing a plurality of values constituting relative measures of focus of the corrected image achieved with each selected dominant scatterer.   
     
     
       7. The method of claim 6 wherein said performing includes: storing the relative measures of focus produced by said repetitions; and   comparing the relative measures of focus to determine the corrected image which is the most focused.   
     
     
       8. The method of claim 7 further comprising: saving the corrected image determined to be the most focused.